Electric machines are generally known to comprise an annular stator and an internal rotor, however different topologies have been already adopted and are actually manufactured.
The stator comprises an iron core provided with slots housing the stator winding. The stator core is made of packets of electrically insulated iron sheets, joined together by thin spacers, which define the cooling channels between the packets for the relevant cooling gas flow.
All stator packets and the spacers are tightened together under pressure by means of press plates at both core ends and additional key bars, generally welded to the core back and to both press plates.
During operation, the stator core can loose its tightness, due to electromagnetic, mechanical and thermal stresses and aging. In particular the iron sheets can start to separate from each other and to vibrate, finally leading to localized hot spots due to short circuits of the sheets and/or to breakdown in the stator winding, i.e. to electric machine failures.
In addition, in case an upgrade (to increase its rated power) or a rewind of the electric machine is foreseen, the stator core conditions must be checked to assess whether it is capable of withstanding the new operating conditions or respectively bearing the expected lifetime extension. The tightness of the stator core is one of the required assessments of the electric machine conditions, which are to be performed before any renewal.
Traditionally, in order to test the stator core tightness, the rotor must be extracted so as to allow enough space within the stator to perform the required tests.
However, rotor extraction is very time consuming and both rewinds and upgrades have strict time constraints for the full implementation. In addition, rotor extraction creates a risk of stator and/or rotor damage.